3D-PDR: A new three-dimensional radiative transfer and astrochemistry code for treating photodissociation regions

3D-PDR3D-PDR: A new three-dimensional radiative : A new three-dimensional radiative transfer and transfer and astrochemistry code for treating astrochemistry code for treating

Photodissociation RegionsPhotodissociation Regions

Thomas G. BisbasThomas G. Bisbas

University College LondonUniversity College LondonHarvard-Smithsonian Centre for AstrophysicsHarvard-Smithsonian Centre for Astrophysics

With thanks to “Friends for

Astronomy Group”, Thessaloniki

Greece

http://ww.ofa.gr

© Kallias Ioannidis

Diffusive Nebulae

© Markos Aspridis

The Orion Nebula (M42)

Diffusive Nebulae


The California Nebula (NGC 1499)

Planetary Nebulae

© Vlachos Paul

© Vlachos Paul

The Dumbbell Nebula (M27)

Ring Nebula (M57)

Supernova Remnants

© Giaourtsis Theodoros

The Crab Nebula (M1)

Supernova Remnants


The Jellyfish Nebula (IC 443)

Structure of ionized regions

© Kallias IoannidisThe Elephant Trunk Nebula

Telescope: AT65QCamera: QHY-9Filters: Hα, OIII, SII, R, G, B.Exp.Time: 12h

Structure of ionized regions


Ionized gas

Massive Stars

Ionization Front

Molecular Gas

Molecular Gas

Ionized region


Emission of ionizing photons (hν > 13.6eV)

Increase the temperature to T ~ 104 K.

Free e- and p+.

MOCASSIN (Ercolano et al. 2003; 2005; 2008) is a three-dimensional algorithm to calculate the chemistry in the interior of an HII region.

Uses Monte-Carlo method.

Telescope: AT65QCamera: QHY-9Filters: Ha,OIII,SIIExp. Time: 390min

Photodissociation Regions (PDR)Neutral regions of the interstellar medium in which the FUV photons strongly influence the gas chemistry and act as the most important source of heat.

PDRs occur in any region of the ISM that is dense and cold enough to remain neutral but has too low column density to prevent the penetration of FUV photons.

PDRs are located in the edge of the HII regions, where the temperature drops very abruptly from T ~ 104 – 10 K.

As the binding energy of the H2 molecule is lower than that of the hydrogen atom, HII

regions are enveloped by a region of atomic hydrogen.

In this region UV is great enough to photodissociate H2 but the recombination rate is

high enough to keep the ionized fraction low.

Deeper in the cloud, UV has been sufficiently attenuated, such that most hydrogen is bound to H

2.

Photodissociation Regions (PDR)

Orion Bar

© Vlachos Paul


Red: emission in CO 1-0 transition

Green: emission in 1-0 S(1) H

2 line

Blue: PAH emission

Tielens et al. (1993)

Photodissociation Regions (PDR)

PDRs are the regions where star formation occurs.

Theoretical studies of PDRs have been done using one-dimensional codes. The chemical structure is very complicated. Effort has been made in understanding the PDRs using one-dimensional astrochemical codes which are able to treat such complicated chemical networks.

However the interstellar medium has been observed to be irregular and to contain many clouds. Therefore, a three-dimensional astrochemistry code is needed in order to examine this arbitrary density distribution.

We have worked in this direction and we have implemented the first three-dimensional code (3D-PDR) which is able to handle such irregular structures.


Dynamical evolution




Offset2.aviBisbas et al. (2009) A&A, 497, 649

Smoothed Particle Hydrodynamics

Triggered Star Formation

Proplyds. Molecular gas surviving UV radiation.

Shock - compressed molecular gas

EGGs


Sequence of Star Formation in the NGC 281 (Pacman Nebula).

Triggered Star Formation – Radiation Driven Implosion

MolecularClump

UV radiation

Cometary tail formed due to compression and re-expansion

Bisbas et al. (2011) ApJ, 736, 142sim2.avi

Bisbas et al. (2011) ApJ, 736, 142

Triggered Star Formation – Radiation Driven Implosion

Modelling Photodissociation Regions: The 3D-PDR code

Overview of the code

The 3D-PDR code uses the chemical model features of the fully benchmarked one-dimensional code UCL_PDR (Bell et al. 2006).

It solves the chemistry and the thermal balance self-consistently within a given three-dimensional cloud of arbitrary density distribution.

The code uses a ray-tracing scheme based on the HEALPix package to calculate the total column densities and thus to evaluate the attenuation of the far-ultraviolet (FUV) radiation into the region, and the propagation of the FIR/submm line emission out of the region.

An iterative cycle is used to calculate the cooling rates using a three-dimensional escape probability method, and heating rates.

At each element within the cloud, it performs a depth- and time- dependent calculation of the abundances for a given chemical network to obtain the column densities associated with each individual species.

The iteration cycle terminates when the PDR has obtained thermodynamical equilibrium, in which the thermal balance criterion is satisfied i.e. the heating and cooling rates are equal to within a user-defined tolerance parameter.

Rays Level of refinement

HEALpix (Gorski et al. 2005, ApJ, 622, 759)



Treatment of the UV radiation field

A realistic treatment of the UV radiation field is needed in order to account for the complicated irregular structures in a three-dimensional HII region. This can be accomplished using the MOCASSIN code to calculate the interior of the ionized region, to obtain a realistic temperature profile and the abundances of species of the given chemical network.

However, we will simplify the calculations by adopting an exponential factor to scale the UV field, neglecting the contribution due to the diffusive radiation and backscattering.

The interstellar radiation field for a given element p(x,y,z) will thus be calculated using the equations:


Gas heating

We account for photoelectric ejections of electrons from dust grains; PAHs; Collisional de-excitation of vibrationally excited H

2 following

FUV pumping; Photoionization of neutral carbon and the energy liberated by the grain surface formation of H

2; Cosmic rays; Turbulence.

Gas Cooling

The gas cooling occurs primarily by the collisional excitation and subsequent emission of a number of key atomic and molecular species. We account for CII, CI, and OI emission, and the rotational transitions of CO

© Vlachos Paul


3D escape probability method

The escape probability method (de Jong, Dalgarno, & Chu 1975, ApJ, 199, 69) describes the probability that a photon of frequency ν

ij escapes from the element

p(x,y,z) without interacting with the rest of the cloud.

An escaping electron cools down the given cloud element since it carries energy.

The three-dimensional approach uses the HEALPix algorithm to calculate each individual escape probability per direction.

Analytical Numerical


Chemistry involved

The code determines the relative abundances of a limited number of atomic and molecular species at each cloud element, by solving the time-dependent chemistry of a self-contained network of formation and destruction reactions.

We use the UMIST database containing 33 species (including e-) and 320 reactions.

We solve for steady-state chemistry (chemical evolution time set to t=100Myr), although the code is able to follow the full time dependent evolution of chemistry within the cloud.

It is thus a powerful tool when one moves to dynamically evolved simulations.


3D-PDR: Benchmarking

3D-PDR: Application 1

Uniform density sphere

UV

Very good agreement with the 1D codes for all models examined

n=103 cm-3, χ=10 Draine

3D-PDR: Application 2

Multi-UV field application

Uniform density sphere

UV

n=5x103 cm-3, χISO

=150 Draine, χUNI

=2000 Draine Cross section of the surface temperature

K k

m /

s

Emission maps for [CII] 158 μm, [CI] 610 μm, [OI] 63 μm, and CO(1-0)

3D-PDR: Application 2Multi-UV field application

RGB composite image for CO(1-0), [CI], [CII] emission maps. The values correspond to the [CII] emission map.RGB colour bar ratios of 5:1:10 for CO(1-0):[CI]:[CII].

3D-PDR: Application 2Multi-UV field application

Simulation using Smoothed Particle Hydrodynamics

RDC.avi

Snapshot at t=0.12Myr from the SPH simulation (see the movie)

3D-PDR: Application 3Radiation Driven Implosion

Emission maps for [CII] 158 μm, [CI] 610 μm, [OI] 63 μm, and CO(1-0)

K k

m /

s



RGB composite image for CO(1-0), [CI], [CII] emission maps. The values correspond to the [CII] emission map.RGB colour bar ratios of 8:1:2 for CO(1-0):[CI]:[CII].

3D-PDR: Projects we are currently developing(with J. Drake, N. Wright & B. Ercolano)

Column density plotRGB synthetic image (CII, CI, CO)

Simulating the PDR of pillar-like structures (see Ercolano et al. 2012, MNRAS, 420, 141).

MOCASSIN synthetic (Hα, OIII)

Simulating PDR and XDR (X-ray dominated region) of disks.

3D-PDR: Projects we are currently developing (with J. Drake, N. Wright & B. Ercolano)

Conclusions

Structure of nebulae (ionized medium, Photodissociation regions)

Triggered star formation in ionized nebulae

Radiation Driven Implosion

3D-PDR code: modelling PDRs in three-dimensions for the first time HEALPix based escape probability method Benchmarking tests agree with the rest of 1D codes Applications in 3D show that 1D codes might not able to simulate properly the clumpy ISM. Test using an SPH snapshot

Future tasks include the coupling of 3D-PDR with MOCASSIN to treat as realistically as possible the irregular structures observed in the Interstellar Medium.

Documents

3D-PDR: A new three-dimensional radiative transfer and astrochemistry code for treating photodissociation regions